U.S. Pat. Nos. 3,620,816 and 4,168,333 describe methods for diffusing elements such as chromium and/or aluminum into the surface of ferrous-based substrates in order to produce an alloyed steel surface. Such methods involve placing the substrate in a molten lead bath containing chromium and/or aluminum for an extended period of time, from one to eighteen hours. The molten lead acts as a transfer agent to transfer the chromium and/or aluminum dissolved in the bath to the substrate and to diffuse the same into its surfaces. Processing times of one hour or more are satisfactory when batch processing is employed; in other words, when the parts are placed in a bath for the requisite time and then removed. Such long periods of time, however, are not satisfactory when it is desired to diffuse chromium and/or aluminum into the surfaces of steel sheet or other products, in coil form, on a continuous basis.
The carbon in the steel while important to giving steel its strength, adversely affects the corrosion resistance of a chromium diffusion layer formed on the surface of the steel. Low carbon steel having a carbon content of between 0.01% and 0.06% by weight is available. Various decarburization heat treatments can reduce the carbon content still further. However, as the chromium diffuses into the surface of such low carbon, decarburized steel, the remaining carbon tends to migrate to the surface. It is believed that when sufficient carbon has diffused into the chromium diffusion alloy layer on the steel, precipitation of chromium carbides will occur during subsequent cooling from the processing temperature, which can result in chromium depletion in regions adjacent to the carbides. This effect is believed to cause the loss in corrosion resistance. Therefore, it is desirable to form the carbon adjacent the surfaces of the ferrous-based material into carbides that are more stable than chromium carbides.
In preparing the bath for performing the diffusing process, chromium is added thereto. Usually chromium is in particulate form and is contained within a cage that is placed in the bath. The cage is normally agitated, causing the chromium particles to dissolve in the molten lead and thereby leave the cage. The chromium is either in elemental form or is an alloy such as ferrochromium. Either form is likely to have debris associated therewith. The debris does not dissolve in the bath and instead travels upwardly in the bath to form a dross on its surface. When sheet steel is drawn through such a bath, it tends to pick up impurities from the dross, Which impurities constitute barriers to diffusion of the chromium. Furthermore, these inorganic particles serve as nucleation sites for the in-situ growth of dendritic structures of alloy crystals that form on the surface. The corrosion resistance of the steel at the sites of the foreign matter is much less. The contamination can be reduced by screening the chromium prior to its being placed in the bath. Additionally, the chromium can be cleaned with chemicals such as solvents and acids.